1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a system and method for forming silicon targets for use in sputter deposition processes.
2. Description of the Related Art
Research continues into methods of producing polycrystalline (p-Si) film by annealing amorphous silicon (a-Si) films. Polycrystalline films are used in the formation of IC active areas, such as the source, drain, and gate regions of a transistor. One specific application is the formation of thin film transistors (TFTs) that are used in the fabrication of active matrix (AM) liquid crystal displays (LCDs).
One relatively recent approach to forming an amorphous silicon film is through silicon (Si) sputter deposition. Little prior art exists in this field, as the application of sputtering method for silicon deposition in microelectronics (i.e. TFTs) is quite new. There is a body of work in the use of silicon targets for sputtering of optical coatings (SiO2, SiNx), but this application differs. The optical coatings are particularly valued for their optical characteristics, not their electrical characteristics. In some circumstances these optical coating targets are heavily doped to improve conductivity of the resulting film. However, a highly doped silicon film cannot be formed into a transistor active area. Furthermore, these optical coatings are a continuous blanket films and, therefore, little regard is paid to particle size and uniformity. Transistor active area silicon films are patterned, however, often to small geometries. Therefore, the particle composition, of little regard in a blanket optical coating film, is critical in the formation of transistor silicon film.
FIGS. 1a through 1e are partial cross-sectional diagrams illustrating the fabrication of a conventional top-gate TFT structure (prior art). Poly-Si (polycrystalline-Si) TFTs are made by a plurality of processes. In the majority of polycrystalline silicon TFT LCD applications, the so-called top-gate, polycrystalline silicon TFT structure is used. Typically, Plasma-Enhanced Chemical Vapor Deposition (PE-CVD) or Low-Pressure CVD (LPCVD) is used to deposit the amorphous silicon precursor. There are several advantages in using physical vapor deposition (PEV) or sputtering to form the silicon film. Such advantages are a reduction in process steps, as there is no need for dehydrogenation, equipment cost reduction, and improved process safety, since no toxic/pyrophoric gases are necessary.
In FIG. 1a a barrier layer 10 is deposited over a substrate 12. Amorphous silicon is deposited over barrier layer 10 and annealed, using an Excimer Laser for example, to form polycrystalline silicon layer 14.
In FIG. 1b the polycrystalline silicon layer 14 is patterned and dry etched.
In FIG. 1c a gate isolation layer 16 is formed over the polycrystalline silicon layer 14. A gate 18 is formed over gate isolation layer 16, and the source region 20 and drain region 22 are implanted with P material.
In FIG. 1d an interlayer dielectric 24 is isotropically deposited.
In FIG. 1e the interlayer dielectric 24 is selectively etched to form vias to the source/drain regions 20/22. A source contact 26 and a drain contact 28 are deposited and patterned. The present invention is concerned with the sputter deposition of the amorphous silicon used to from polycrystalline silicon layer 14 (FIG. 1a).
FIG. 2 is a partial cross-sectional diagram of a typical DC magnetron sputtering chamber (prior art). One of the key aspects of the silicon-sputtering process is the ‘target’ component. The target is a block of the material to be deposited, mounted on an appropriate metal backing plate, and placed opposite to the substrate where the film is to be deposited. Plasma strikes in the gap between the target and the substrate. The magnet that is scanning above the target backing plate is used to intensify the plasma and confine it in the region defined by the magnetic field. By scanning the magnet, the plasma is swept across the surface of the target, resulting in deposition of the film on the substrate opposite to the target. The plasma is generated by applying high voltage to an inert gas (typically Ar, but alternately He, Ne, Kr or mixtures) that flows in the region between the target and the substrate. For certain applications, other gases may be mixed to the sputtering gas, such as H2, O2, N2, etc., to alter the composition and/or the properties of the sputtered film.
The target is an important component of the sputtering process because it affects the level of contamination in the film, as well as the level of particles, which are generated during the deposition process. Particles are fragments of silicon material that are detached from the target material during processing. Particles larger than approximately 5 microns are not desirable in TFT process films. Hence, the target manufacturing must proceed in a way that the produced target can yield low levels of contamination in the deposited film as well as a low level of particles. High particle levels result in low yields, as well as reduced equipment up-time, since frequent cleaning of the chamber is required. Film contamination needs to be reduced below acceptable levels, for the films to be suitable for the fabrication of electronic devices.
The issue of particles is particularly severe for silicon targets for two reasons: (1) the target is a tiled assembly (not a single piece), which means that the edges of the tiles can generate particles, unless they are properly prepared; and, (2) the target material is generally of low resistivity (does not conduct thoroughly) as a result of the purity requirements of the deposited film. Hence, the material is susceptible to charge buildup and, consequently, arcing, especially if the surface of the target is not properly conditioned. Arcing may result in increased contamination in the film, especially with the material that the chamber and internal components being made of Al, Ni, or equivalent metals.
It would be advantageous if a process existed for efficiently sputtering silicon to form an amorphous silicon film.
It would be advantageous if the amorphous silicon film could be sputter deposited without particle contaminants.
It would be advantageous if silicon targets existed that minimized contamination in the sputter deposition of amorphous silicon film.